A method for making an electrophotographic plate

ABSTRACT

In an electrophotographic camera, a photon image to be recorded is focused through an optically transparent substrate and transparent electrode onto the back surface of a photoconductive layer. The charge-retentive surface of an electrographic recording paper is disposed adjacent the photoconductive layer and the conductive backing of the paper is connected to an electrode for impressing a charge transfer potential across the photoconductor layer and the charge-retentive layer of the paper. When the potential is impressed across the photoconductor, electrons liberated in the photoconductor by the photon image to be recorded are transferred to the charge-retentive surface of the recording paper to form a charge image of the object to be recorded. The charge image is subsequently developed by applying charged toner particles to the image for developing same. The photoconductive layer comprises a substantially continuous layer of an interlocked matrix of crystals of active photoconductive material coated and bound together with a lead sealing glass interstitially disposed of the interlocking crystal matrix. The resultant photoconductive layer has improved strength and resistance to abasion while producing acceptable photographic images. The photoconductive layer is produced by heating a stratum including particles of lead sealing glass together with particles of preactivated photoconductive material in the presence of a molten solvent and proportions of one or more activators.

United States Patent [191 Mooney 1 Aug. 28, 1973 METHOD FOR MAKING AN ELECTROPHOTOGRAPHIC PLATE [75] Inventor: John B. Mooney, Saratoga, Calif.

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Apr. 5, 1971 [21] Appl. No.: 131,021

[52] U.S. Cl 117/34, 117/45, 117/169, 117/201, 117/211, 117/217, 96/1 R, 250/3.3,

[51] Int. Cl. 844d 1/46, G03g 5/00, 603g 7/00 [58] Field of Search 117/34, 45, 169, 117/201, 211, 217; 250/8, 33; 252/301;

[56] References Cited UNITED STATES PATENTS 3,288,603 ll/l966 Corrsin 96/1 3,248,261 4/1966 Narken et al. 117/34 2,765,385 10/1956 Thomsen 117/45 2,857,541 l/1958 Etzel et a] 252/3016 2,698,915 l/1955 Piper 1l7/33.5 2,866,117 ll/l958 Walker et al..... 117/215 2,937,353 5/1960 Wasserman 117/215 3,151,982 10/1964 Corrsin 117/34 Primary Examiner-William D. Martin Assistant ExaminerWilliam R. Trenov Attorney-Leon F. Herbert and Paul Hentzel [57] ABSTRACT In an electrophotographic camera, a photon image to be recorded is focused through an optically transparent substrate and transparent electrode onto the back surface of a photoconductive layer. The charge-retentive surface of an electrographic recording paper is disposed adjacent the photoconductive layer and the conductive backing of the paper is connected to an electrode for impressing a charge transfer potential across the photoconductor layer and the charge-retentive layer of the paper. When the potential is impressed across the photoconductor, electrons liberated in the photoconductor by the photon image to be recorded are transferred to the charge-retentive surface of the recording paper to form a charge image of the object to be recorded. The charge image is subsequently developed by applying charged toner particles to the image for developing same. The photoconductive layer comprises a substantially continuous layer of an interlocked matrix of crystals of active photoconductive material coated and bound together with a lead sealing glass interstitially disposed of the interlocking crystal matrix. The resultant photoconductive layer has improved strength and resistance to abasion while producing acceptable photographic images. The photoconductive layer is produced by heating a stratum including particles of lead sealing glass together with particles of preactivated photoconductive material in the presence of a molten solvent and proportions of one or more activators.

6 Claims, 3 Drawing Figures I OPTICALLY TRANSPARENT CONDUCTOR MATRIX or INTERLOCKING PC PC PC P6 50 83 PHOTOCONDUCTOR CRYSTAL PARTICLES Patented Aug. 28,1973 3,754,965

FIG.| 4

I I3 H 5 5 Mfi/j/jWfi Yffl HIXUNACTIVATED PHOTO- conoucron POWDER,ACTIVATOR, GLASS SUBfTRATE AND SOLVENT m A VEHICLE F f TOMAKEAPASTE //l DRY OPTICALLY/N, T'

TRANSPARENT CRUSH AND BLEND CONDUCTOR l6 PREACTIVATINGFIRING V I6 5 GLASS CRUSH MATRIX or mx WITH EXTRA SOLVENT AND L ,$5T,%

cuss INAVEHICLETO CRYSTAL FORM A PASTE PART'CLES APPLY TO SUBSTRATE F|(; 3

FIREINAIR TOSINTER PHOTO- JOHN B 8551? CONDUCTOR,DRIVEOFFUNUSED 8Y SOLVENT, AND FUSE GLASS M mro MATRIX fl METHOD FOR MAKING AN ELECTROPIIOTOGRAPHIC PLATE DESCRIPTION OF THE PRIOR ART Heretofore, photoconductive layers have been produced by forming a stratum including particles of a material selected from the group consisting of sulphides, selenides and sulphoselenides of cadmium, recrystallizing said material in a molten solvent to a desired range of particle sizes, incorporating into said recrystallized material activator proportions of a halide and activator proportions of a metal selected from a group consisting of copper and silver, and evaporating the molten solvent. The resultant layer is a substantially continuous layer of interlocked crystals of photoconductive material. Such a sintered photoconductive material is described in U.S. Pat. No. 2,765,385, issued Oct. 2, 1956.

The problem with this prior art photoconductive material is that it is relatively fragile and non-resistant to abrasion, thereby precluding its use as a camera plate in an electrographic camera. The abrasion of the photoconductive surface in such an application would produce surface scratches and transfer of the photoconductive layer to the electrophotographic recording paper, thereby rendering the photoconductive camera plate unusable.

Others have constructed xerographic plates of copper and chlorine-doped cadmium sulphide photoconductive powder incorporated in an acrylic resin consisting of n-butyl and isobutyl methacrylate. The acrylic resin served as a binder for the photoconductive particles and comprises approximately 1 1 percent by weight of the layer. The layer was formed by mixing the photoconductive powder and the acrylic resin in xylene to form a slurry. The slurry was applied to a tin oxide coated borosilicate glass substrate and air-dried and further dried at 80 C for several hours. The resultant photoconductive layer had improved mechanical stability over the aforementioned photoconductive layer but was found to have greatly reduced ASA speed, as of l 1 th of that of the aforedescribed prior art plate. Such a photoconductive plate is described in an article titled Photoinduced Discharge Characteristics of Cadmium Sulphide Binder Layers in the Xerographic Mode appearing in the Journal of Applied Physics, Vol. 36, No. 11, of Nov. I965, pp 3475-3480. The problem with using such a photoconductor as an electrographic camera plate is that it produces rather speckled images due to a lack of uniformity of the resultant layer, it has a relatively high dark background current, and is sensitive to the presence of moisture which tends to alter its photoconductive properties and to give rise to excessive dark conductivity. In addition, this latter type of photoconductive plate suffers from memory effects, thereby precluding its use in a camera wherein the time interval between successive picture frames is desired to be as short as possible.

Therefore, the need exists for an improved photoconductive plate which will have relatively high ASA speeds, will be mechanically strong and resistant to abrasion, and which will be highly homogeneous and free of surface blemish and defects while providing relatively little memory and having very low dark current.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved photoconductive layer and method of making same.

One feature of the present invention is the provision of a sintered photoconductor comprising a layer of a substantially continuous polycrystalline matrix of interlocked photoconductive crystals of a substance selected from the group consisting of sulphides, selenides, tellurides and sulphoselenides of a member of the group consisting of zinc and cadmium, and containing activator proportions of a halide plus activator proportions of a metal selected from the group consisting of copper and silver and incorporating a glass binder interstitially disposed of said polycrystalline matrix, whereby a mechanically stable abrasion-resistant, highspeed photoconductive layer is formed having low memory and low dark current characteristics.

Another feature of the present invention is the same as the preceding feature wherein the photoconductive crystals comprise crystals of cadmium sulphide containing activator proportions of chloride and copper and wherein the glass binder is a lead sealing glass.

Another feature of the present invention is the same as any one or more of the preceding features wherein the photoconductive layer is produced by a method including the steps of, forming a mixture including particles of solvent and activator mixed together with particles of a substance selected from the group consisting of sulphides, tellurides, selenides and sulphaselenides of a member of the group consisting of zinc and cadmium, activating said substance, mixing the activated substance with glass particles, forming a layer of said activated substance and glass mixture, recrystallizing at least portions of said substance in said layer and melting the glass particles which.are interstitially disposed of said activated photoconductive substance.

Another feature of the present invention is the same as the preceding feature wherein the glass particles have a softening temperature between 50 and 250C below the temperature at which the mixed particles are heated for melting the solvent.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram, partly in section and partly in block diagram form, depicting an electrophotographic camera incorporating features of the DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown an electrophotographic camera 1 incorporating features of the present invention. The camera 1 includes a lens 2 disposed at one end of a dark box 3 for focusing the light obtained from an object 4 onto the back side of a photoconductive layer 5 disposed at the image plane of the lens 2. The photoconductive layer 5, as of 20 to I microns, is deposited over an optically transparent conductive electrode 6 which in turn is supported from an optically transparent substrate 7, as of borosilicate glass plate one-fourth thick. A suitable transparent conductive electrode structure 6 comprises a tin oxide coating having a resistivity of 50 ohms per square and having a transparency, in the optical range, greater than 95 percent. Other suitable conductive electrodes 6 include metal films of chromium and gold.

An electrographic recording paper 8 is disposed adjacent the photoconductive layer with the chargeretentive surface 9 of the paper 8 disposed adjacent the photoconductive layer 5. The conductive layer 11 of the paper 8 is disposed facing a conductive electrode structure 12 for uniformly pressing the chargeretentive surface 9 of the paper 8 into nominal contact with the surface of the photoconductor 5. A source of potential, as of 500 to 900 volts, is connected across electrodes 6 and 12 via the intermediary of a timing switch 14. An electrographic camera 1 of the type herein disclosed is described and claimed in copending U.S. application Ser. No. 599,069 filed Dec. 5, I966 and assigned to the same assignee as the present invention, now US. Pat. No. 3,502,408.

In operation, the image of the object 4 to be photographed is focused upon the photoconductive layer 5. The timing switch 14 is closed for the appropriate ex posure time determined by the available light intensity and the speed of the photoconductive layer 5. During the exposure time, electrons liberated within the photoconductive layer 5 by the incident light image are caused to be transferred through the photoconductive layer 5 into the charge-retentive surface 9 of the electrographic paper 8. In this manner, a charge image of the object 4 is produced in the charge-retentive surface 9 of the paper. The charge image is then developed by removing the paper 8 from the camera 1 and applying positively charged toner particles to the charge image for developing same. The toner particles may be suspended in air or in a liquid dielectric vehicle. Alternatively, the polarity of the source 13 may be reversed to produce positive charge images on the charge retentive surface 9.

The photoconductor 5 is also sensitive to invisible radiation; i.e., photon of energy outside the visible range of wavelengths. For example, the photoconductor is useful for photographing X-ray, X-ray or neutron images. In such latter applications, the transparent electrode 6 and substrate 7 need only be transparent to the rays which are to form the image on the photoconductor 5.

Referring now to FIG. 2, there is shown the structural detail of the photoconductive layer 5 of the present invention. The photoconductive layer 5 comprises a substantially continuous polycrystalline layer of a matrix of interlocked photoconductive crystals 15 of a photoconductive substance selected from the group consisting of sulphides, tellurides, selenides, and sulphoselenides of a member of the group consisting of zinc and cadmium and containing activator proportions of a halide plus activator proportions of a member of the group consisting copper and silver. A lead sealing glass binder material 16 is interstitially disposed of the polycrystalline matrix 15, thereby binding the matrix together to make the matrix mechanically stable and resistant to abrasion. Each of the photoconductive crystallites of the matrix 15 is fused to its neighboring photoconductive crystallites by means of a recrystallized bridge or junction therebetween, thereby producing low resistance electrical bridging connections between neighboring photoconductive crystallites of the matrix forming the photoconductive layer 5.

The surface of the photoconductive layer 5 which faces the charge-retentive surface 9 of the paper 8 is relatively smooth, having a surface ripple less than 5 microns. The surface ripple is defined as the vertical distance between adjacent peaks and valleys in the surface of the photoconductor. Although the glass binder 16 completely coats the surface of the photoconductive particles 15 at the exposed surface of the photoconductive layer 5, the thicknes of the glass is only on the order of a micron or less and does not interfere with proper operation of the photoconductive layer 5. Photoconductive layers 5 produced in accordance with the teachings of the present invention are found to be resistant to abrasion, and to be mechanically strong for producing electrographic images of acceptable photographic quality; i.e., defects cannot be discerned by the unaided eye.

Referring now to FIG. 3, there is shown a flow diagram, in block diagram form, depicting the method for fabricating photoconductive layers 5 according to the present invention. Briefly, the method for producing the photoconductive layer 5 comprises the steps of: mixing together the powdered photoconductor and a suitable solvent or fusing agent and a suitable activator; firing the mixture at about 600C for about 25 minutes; mixing powdered glass into the fired mixture; dispersing the mixture into a suitable vehicle to form a paste; applying the mixture by a doctor blade or by spraying or by painting onto the transparent conductive electrode layer 6 as carried upon the glass substrate 7; firing the substrate containing the coating in an air furnace and sintering at 600C for approximately 15 minutes; cooling the resultant sintered photoconductive layer to room temperature. A conductor is then painted onto the side edge of the substrate member to make contact with the optically transparent conductive electrode 6 and the photoconductive plate is then ready for use.

Suitable photoconductive materials include sulphides, tellurides, selenides and sulphoselenides of zinc or cadmium. Suitable activator elements include a halide plus copper and/or silver. Suitable solvents for the photoconductor include halides of cadmium or zinc. Suitable vehicles for the powdered mixture include ethyl alcohol and xylene. Suitable glasses include the lead sealing glasses having a softening temperature between 50 and 250C below the temperature at which the mixed particles are heated for melting the solvent. The glass particles preferably comprise between 10 percent and 45 percent by weight of the particulated photoconductive substance exclusive of the glass particles. In a preferred embodiment, the sealing glass has a softening point of approximately C below the temperature at which the particulated layer is fired in the furnace.

In a specific example of a method for forming the photoconductive layer 5 of the present invention, an intimate mixture is formed of 250 grams of cadmium sulphide, 0.25 grams of Cu CL 2 ZI-I O and 12.5 g of dry Cd cl- 2.5H2O. A suitable cadmium sulphide powder is high purity powder obtained from Gallard Schlesingler Chemical Company of Carle Place, N.Y.. Batch No. B7649. The cadmium chloride and copper chloride are predissolved in deionized water. and sufficient water is added to make a fluid paste (approximately 150 milliliters total). The slurry is thoroughly blended (e.g.. by hand-mixing, ball-milling, etc.), then dried (e.g.. over a hot-plate or in an oven). The dry material is pulverized, e.g., in a mortar and pestle until all of it passes through a -mesh sieve, blended and placed in a glassceramic dish. The mix is fired at 600C for minutes. then cooled and again crushed until all of it passes through a 50-mesh screen. This is the preactivated photoconductor powder. To 250 grams of preactivated cadmium sulphide, one adds another 12.5 grams of cadmium chloride and 50 grams of Coming No. 7570 glass powder with a particle size 325 mesh (coming Glass Works, Corning, N.Y.). The mix is slurried up in a 16 oz. glass jar with about 200 milliliters of deionized water, cylindrical milling balls as X /2 inch) are added, and the slurry is milled for at least hours. If necessary, more water may be added to give a mix of the desired consistency.

During the preactivating firing step, the cadmium chloride melts, dissolving the copper salt and some of the cadmium sulphide. In the molten solution, ion exchange chemical reaction takes place. In these chemical reactions, copper activates the photoconductive cadmium sulphide material. In addition, an ion exchange reaction occurs wherein chlorine ions enter the cadmium sulphide lattice to produce further activation of the photoconductive material.

On further heating, the cadmium sulphide recrystallizes at the junctions between adjacent cadmium sulphide crystals and the unused cadmium chloride evaporates. The recrystallized cadmium sulphide has incorporated therein activator proportions of copper and chlorine. During plate firing, the additional cadmium chloride acts as a fusing agent for producing conductive bridging connections between the adjacent crystallite particles of the photoconductive material. When substantially all of the unused cadmium chloride has evaporated, the cadmium sulphide crystals are interlocked with one another, forming a substantially continuous polycrystalline layer of interlocked photoconducting crystals on the glass plate. The resultant layer is extremely homogeneous and firmly adherent to the glass.

The lead sealing glass has a softening temperature of approximately 150 lower than the firing temperature of 600. The sealing glass does not appear to react with the preactivated photoconducting crystals. The glass, upon melting, forms a coating around the photoconductor and provides a binder filling the interstitial spaces between the inerlocked crystallites of the photoconductive matrix. The glass imparts mechanical strength and abrasion-resistant characteristics to the resultant photoconductive layer 5.

In place of cadmium sulphide, sulphides, tellurides, selenides and sulphoselenides of zinc or cadmium may be used. Cadmium sulphide and its equivalents will hereinafter be referred to as the host crystal.

During preactivation cadmium chloride is introduced into the mixture to act as a solvent for the host crystal.

In addition to cadmium chloride, other halides of cadmium or zinc may be used such as, for example, bromides or iodides of cadmium or zinc may be employed. lnstead of copper, silver may he introduced into the host crystal as the activator. The proportion of glas by weight of the host crystal preferably falls within the range of l0 percent to 45 percent with 20 percent being especially desirable.

Since many changes could be made in the above consruction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A method for manufacturing an electrophotographic plate for transferring developable quantities of charge to an adjacent surface in response to photons incident upon the plate, comprising the steps of:

forming a mixture of crystals, a solvent for the crystals, and an activator suitable for promoting photoeonductivity in the crystals, the crystals selected from the group consisting of sulphides, tellurides, selenides, and sulphoselenides of a material selected from the group consisting of zinc and cadmium;

firing the mixture to melt the solvent and activate the crystals;

cooling the fired mixture;

adding particles of glass to the fired mixture wherein the glass particles comprise from about 9 percent to about 31 percent by weight of the weight of the fired mixture including the weight of the glass; providing a sustrate;

forming a stratum on the substrate with the fired mixture; firing the stratum to melt at least a portion of the solvent which dissolves at least a portion of the crystals, while simultaneously melting the particles of glass to interstitially coat the crystals; and

lowering the temperature of the stratum to cause the dissolved crystals to recrystallize establishing conductive bridges between the particlesof crystals and to cause the glass to solidify and bind the crystals together for use in an electrophotographie plate for transferring charge to the recording medium.

2. The method of claim 1, wherein the crystals are cadmium sulphide.

3. The method of claim 1 wherein the activator is a halide and of a metal selected from the group consisting of copper and silver, and the solvent is a halide of cadmium.

4. The method of claim 3 wherein the activator is chloride and copper.

5. The method of claim 1, wherein the particles of glass comprise about 23 percent by weight of the total weight of the fired mixture including the glass particles.

6. The method of claim 1, wherein additional solvent is added to the fired mixture before the stratum is formed. 

2. The method of claim 1, wherein the crystals are cadmium sulphide.
 3. The method of claim 1 wherein the activator is a halide and of a metal selected from the group consisting of copper and silver, and the solvent is a halide of cadmium.
 4. The method of claim 3 wherein the activator is chloride and copper.
 5. The method of claim 1, wherein the particles of glass comprise about 23 percent by weight of the total weight of the fired mixture including the glass particles.
 6. The method of claim 1, wherein additional solvent is added to the fired mixture before the stratum is formed. 